Under the influence of hormones, the mammary gland undergoes marked developmental changes to express its differentiated function during lactation. The synergistic actions of hormones such as prolactin, glucocorticoid and insulin can stimulate the functional differentiation of murine mammary epithelium by inducing the expression of milk protein genes as well as other types of gene involved in lactogenesis. This system serves as a good model to elucidate the molecular mechanisms by which hormones regulate developmental and tissue-specific gene expression in mammalian cells. Previous studies show that the lactogenic hormones stimulate transcription of milk protein genes by inducing specific transcription factors. During the course of our study to clone and characterize transcription factors involved in mammary cell differentiation, we have discovered and cloned a cDNA which encodes a transcription activating factor for the murine selenocysteine tRNA gene and named this mStaf. The selenocysteine tRNA is essential for the biosynthesis of selenoprotein as a donor of selenocysteine by which selenium exerts it biological action. By transfection experiments we demonstrated that mStaf plays a critical role in regulation of selenocysteine tRNA gene transcription. Our data also showed that the level of mStaf in the mammary gland increases markedly in parallel with the increase of selenocysteine tRNA during pregnancy and lactation when the gland undergoes growth and development.To investigate the regulation of mStaf binding and its role in selenocysteine tRNA gene expression, we examined the effect of lactogenic hormones on mStaf gene expression in the mammary gland. Our data indicate that the lactogenic hormones increase the level of mStaf transcript and protein through the MAPK pathway and enhance the selenocysteine tRNA levels in the mammary gland.